The isolated perfused rat brain preparation consists of the skull of a rat and its contents with practically all extra-cranial tissue removed. This isolated brain preparation is made from a rat which was adequately anesthetized by deep hypothermia without the use of a chemical agent for anesthesia. This avoids possible interference by residual anesthetic agent on subsequent measurements of activities of the brain. In such a preparation the intensity and density of catecholamine fluorescent varicosities and serotonin cell bodies are essentially similar to that observed in the non-perfused brain taken from rats immediately following decapitation. Perfusion of the isolated brain with methadone or morphine resulted in a decrease in the vasopressin content of the supraoptic nucleus.